Method for in-furnace reduction and control of sulfur trioxide

ABSTRACT

A method for controlling SO 3  in a combustion process of a sulfur-containing fuel, the method steps including partially combusting the fuel to create a reducing environment; maintaining the reducing environment for a sufficient period such that SO 3  is reduced to SO 2  to achieve a desirable level of SO 3 ; and combusting the remainder of the fuel in an oxidizing environment; thereby reducing the conversion of levels of SO 3  in the flue gases.

CROSS-REFERENCE TO RELATED APPLICATIONS

This nonprovisional utility patent application claims the benefit of aprior filed provisional application 60/544,724 filed Feb. 14, 2004,which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method for reducingbyproducts emissions from combustion reactions, and, more particularly,to a method for reducing sulfur trioxide (SO₃) in combustion furnaces.

2. Description of the Prior Art

SO₃ Decrease

The flue gas of power generation plants has long been recognized as asource of atmospheric pollution. In the combustion of fossil fuels, someof the naturally present elements are oxidized to form acids, such asSO₃, NOx, HCI, HF, and the like. These acids, especially SO₃, can becomea problem if their concentrations exceed certain thresholds. Forexample, as the SO₃ concentration increases, the acid dewpointtemperature of the flue gas increases. If the temperature of the fluegas is less than the acid dewpoint temperature of the flue gas, the SO₃in the gas will condense and react with water to form H₂SO₄, causingcorrosion problems inside the furnace. Also, flue gases exiting afurnace cool immediately and SO₃ and other acids in the gas condense,creating localized acid rain, which is the condensation andprecipitation of SO₃ and other acids onto the surrounding land withsubsequent corrosion. Excessive SO₃ will condense into small droplets,creating a visible plume as it exits the furnace, which becomes anesthetic and local political problem. If NH₃-like compounds are presentin the flue gas, they can react with SO₃ to form ammonium bisulfate(NH₃HSO₄) which then fouls the air heater.

Thus, a need exists to decrease the acid dewpoint temperature of theflue gases such that the acid dewpoint temperature is lower than theflue-gas temperature in the coolest parts of the furnace, such as theducts and stack. A further need exists to lower the acid content of theflue gases such that the localized acid rain and other problemsassociated with high-acid flue gas are minimized.

SO₃ Increase

The particulate matter carried in the flue gas can be removed byelectrostatic precipitators that cause the individual particles toaccept an electrical charge and then use that charge to attract them tocollector plates for disposal. The efficiency of such electrostaticprecipitators is dependent upon the ability of the individual particlesto take a charge, that is, the resistivity of the particles. It has beenfound that the presence of SO₃ in the flue gas effectively reduces theresistivity of the particles, making them easier to chargeelectrostatically.

In the combustion of coal, some of the naturally present sulfur isconverted to SO₃. On the other hand, the effectiveness of SO₃ inreducing the resistivity of the particulate matter in the flue gasdepends upon the concentration of the SO₃, with about 15 to 20 parts permillion (ppm) giving optimal results. Therefore, precipitator efficiencyis affected by the ability to adjust the amount of SO₃ in the flue gas,regardless of the sulfur content of the coal being burned, to provide anoverall SO₃ concentration in the optimal range.

SO₃ is also produced in SCR (catalyst) installations by the oxidation ofSO₂ and often exceeds the optimal 15 to 20 ppm optimal concentrations.The catalyst blends typically used in the SCR to reduce NOx to N₂ (inthe presence of ammonia) also oxidize SO₂ to SO₃. The rate of thisreaction is strongly temperature dependent and, at higher temperatures,can convert more than 1 percent of SO₂ to SO₃. High sulfur U.S. coalgenerates anywhere from 2,000 to 3,000 ppm of SO₂ in the boiler, andtherefore can result in 20 to 30 ppm of SO₃ out of the SCR. The problemis that as much as 50 percent, or 10 to 15 ppm, of the SO₃ coming out ofthe SCR will make it past the scrubber and out of the stack. At about 8to 10 ppm, depending upon the particulate concentration, SO₃ becomesvisible as a blue plume.

Furthermore, SO₃ can also be produced catalytically on other boilersurfaces through interaction with elements/chemicals such as Vanadium.

Therefore, because any SO₃ formed prior to the SCR adds to the effluentSO₃, reducing the SO₃ formed prior to the SCR is important for reducingthe effluent SO₃ and permits the use of SCR for the reduction of NOx forgases without generating excessive amounts of SO₃.

SO₃ Control

If the SO₃ concentration is too low, the precipitator will operate atless than optimal efficiency. On the other hand, if the SO₃concentration is too high, the flue gas becomes highly acidic, creatinga “blue plume” and contributing to acid rain. In addition, acidic fluegases contribute to corrosion of the pipes carrying the flue gas, and,when combined with NH₃-type chemicals, can clog the air heater.

Furthermore, an SCR is often only intended to be used for six months peryear (during the summer ozone control season), and are bypassed duringthe winter. This creates seasonal variability in the SO₃ concentrationsat the precipitator, in the duct work, and out of the exhaust stack.

It is therefore desirable to control the concentrations of SO₃ in theflue gas depending upon whether the SCR is in use or not. SO₃concentrations approaching 40 ppm produce severe adverse local acidproblems that are not necessarily regulated, but create local politicalproblems for the facility. The U.S. EPA has indicated that futureregulations on SO₃ emissions are to be expected.

It is desirable, therefore, to have an SO₃ flue gas system that iscapable of adjusting the concentrations of SO₃ in a flue gas with orwithout an SCR installed to maintain the SO₃ concentration at an optimallevel for increased ESP performance, without increased localized SO₃emissions.

Staging

Combustion staging is the process of burning a fuel, i.e., coal, in twoor more stages. A fuel-rich stage, or simply, rich stage, is one inwhich not enough air is available to fully burn the fuel. A fuel-leanstage is one in which there is sufficient or extra air to fully burn thefuel. Staging is used in the prior art to reduce NOx by a) reducing peaktemperatures (thermal NOx) and b) providing a reducing environment (NOxreduction). Macro-staging is the dividing of whole sections of a furnaceinto rich and lean stages and is accomplished through the use of suchtechniques as Over-Fired Air (OFA). Micro-staging is the creation ofproximal microenvironments with functionally different characteristics,such as reduction potential, temperature, and the like. Micro-staging ina furnace can be achieved, for example, in the first stage of thefurnace through the use of Low-NOx burners with adjustment of spin-vanesettings and registers. Increased staging increases the residence timein a reducing atmosphere and increases the effect of the reducingatmosphere.

Prior art has used micro-staging to reduce NOx emissions in combustionfurnaces. Low-NOx burners (LNB) stage by delivering high-fuel-contentprimary air into the furnace that mixes with secondary air flowingthrough one or more secondary air registers. LNB primarily usemicro-staging. The flow through a LNB is designed such that the volatilecomponents of the coal mix with the available near-field air at astoichiometric ratio near unity (1.0), thus anchoring the flame. The netcombustion in the central core near the burners is overall fuel rich anddoes not produce much thermal NOx, as the temperatures are low. The coalis eventually consumed over the depth of the furnace as more and moreair slowly mixes into the central core. The majority of the NOx createdin this region is from the fuel-bound nitrogen reacting to NO throughthe intermediate HCN. The rate at which the outer secondary air mixesinto the core flow is set by the dampers and the spin vanes, as well asthe spin vane in the coal pipe. LNB systems decrease NOx by stagingsince there is a continuous mixing of the rich products of combustionand secondary air throughout the combustion zone. Staging is increasedby decreasing the mixing rate between the rich core flow and the outersecondary air flow.

Prior art has used macro-staging to reduce emissions in combustionfurnaces. Macro-staging consists of highly mixed fuel and air in thelower furnace, mixed to a stoichiometric ratio below unity for a largepart of the flow. Excess oxygen is ultimately required to assure thatall of the fuel has burned and to reduce explosion risks. In amacro-staged furnace, excess air is introduced downstream of theburners. Increased staging is achieved by increasing the residence time,temperature, or reducing quality of the combustion products in theabsence of oxygen.

Prior art used both micro-staging (LNB) and macro-staging (OFA) toreduce NOx emissions in combustion furnaces. In the case of bothmicro-staging and macro-staging, components of each of the above areused and adjusted to achieve NOx emissions reduction.

Staging has nowhere been taught in the prior art for flue gas acidityreduction, acid dewpoint temperature control or SO₃ concentrationcontrol in combustion gases.

SUMMARY OF THE INVENTION

The present invention is directed to a method for reducing SO₃ incombustion systems and methods.

The present invention is further directed to a method for controllingSO₃ in combustion systems and methods.

The present invention is still further directed to a combustion furnacethat uses methods for reducing SO₃ in combustion systems and methods.

It is therefore an object of the present invention to provide a methodfor reducing SO₃ in combustion systems and methods using combustionstaging.

Another object of the present invention is to provide a method forcontrolling SO₃ in combustion systems and methods using combustionstaging.

It is another object of the present invention to provide a combustionfurnace that uses methods whereby SO₃ can be controlled and the methodcan adapt to variations in the sulfur content of the fuel being burned.

These and other aspects of the present invention will become apparent tothose skilled in the art after a reading of the following description ofpreferred embodiment(s) when considered with the drawings.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views. Also in thefollowing description, it is to be understood that such terms as“forward,” “rearward,” “front,” “back,” “right,” “left,” “upwardly,”“downwardly,” and the like are words of convenience and are not to beconstrued as limiting terms. In the present invention, “reducible acid”refers to acids in which the acidity can be reduced or eliminated by theelectrochemical reduction of the acid.

The present invention is directed to a method for in-furnace decreaseand control of the acid dewpoint temperature using combustion staging.The present invention is further directed to a method for in-furnacereduction and control of SO₃ using combustion staging. Increased stagingis advantageously used to simultaneously decrease the acidity, decreasethe acid dewpoint temperature and reduce the SO₃ levels of the flue gas.

Acidity, Acid Dewpoint Temperature and SO₃ Reduction throughMicro-Staging

Similar to how NOx is reduced back to N₂ in a rich “reducing”environment, SO₃ is reduced back to SO₂ in a reducing environment. Withmacro-staging, the center of the furnace below the OFA ports is largelyfuel-rich. This staged environment can be adjusted to be even less mixedto create reducing micro-stages within the first stage of the furnace.The mixing can be reduced by reducing the spin vane velocity settings ofthe primary air and coal flow, or additionally or otherwise, thesecondary air spin vane and register settings, thus creating reducingmicro-stages, or additionally or otherwise, the relative exit velocitiesbetween the primary air and coal flow and the secondary air flow can bechanged.

While the majority of fuel-bound sulfur forms SO₂, some forms SO₃directly during combustion of the fuel-bound sulfur. SO₂ can react toform more SO₃ through the following oxidative reaction:SO₂+O(+M)→SO₃(+M)

However, this three-body reaction is very slow. An additional source forSO₃ production in an oxidative environment is through the reaction:SO₂+O₂→SO₃+O

This reaction does not require three bodies to simultaneously collide;however, it is very sensitive to temperature, requiring hightemperatures, and it is susceptible to reverse reaction:SO₃+O→SO₂+O₂

None of the above three reactions occur in any significant quantity in areducing environment due to the lack of O and O₂ species. In a reducingenvironment, the direct conversion of SO₃ to SO₂ takes place through thefollowing general “reducing” reaction:SO₃+R→SO₂+RO

Where R is any reducing radical species. The primary radical in fossilfuel combustion is the H radical.SO₃+H→SO₂+OH

Many radicals and molecules can be functional in a reducing environment;e.g., H, OH, C, CO, CH, CH₂ C₂H, CH₃, C_(n)H_(m), N, NH_(i), and manyothers.

The above SO₃ reducing reactions are very fast when there aresignificant concentrations of the reducing radicals (“R”). Sufficientconcentrations exist primarily in reducing environments within the first(fuel-rich) stage of the furnace.

In a fuel-rich staged, reducing environment, oxidative chemistryterminates due to greatly decreased (extinguished) concentrations ofoxidative combustion species; e.g., OH, O, O₂, HO₂, H₂O₂, and manyothers. In this environment, species are very competitive for anyavailable oxygen species. Molecules with oxygen atoms that exist inrelatively small concentrations are consumed by oxygen-wanting speciesthat exist in high concentration; e.g., the oxygen in NO is consumed byother species like C, CO, H, and CH. Molecules that have multiple oxygenatoms are particularly at risk; i.e., SO₃ will quickly revert to SO₂through oxygen abstraction by just about anything around, most notably Hatoms.

Thus, in a reducing environment, the SO₃ reduction reaction is very fastvirtually irreversible while the reducing environment is maintained.

Surprisingly and importantly, with the present invention methods andsystems, the net effect is that any SO₃ that is formed during combustionis quickly reduced to SO₂ in the first stage and SO₃ is not reformed byoxidation to SO₂ because there is not enough residence time atsufficiently high temperature in the furnace in the latter, fuel-leanstages. Thus, the present invention advantageously uses the differencesin reaction rates to reduce and maintain the SO₃ levels in the flue gas.

Increased staging increases the residence time in a reducing atmosphere,or increases the reducing potential of the atmosphere, to decrease SO₃concentration and thereby lower the dewpoint temperature. Therefore, toincrease the reduction of SO₃, the residence time can be increased orthe reducing potential in the flue gases can be increased.

To increase residence time, several methods are available:

1) The distance between stages can be lengthened;

2) The mixing can be increased for macro-staging applications;

3) The mixing can be decreased for micro-staging applications;

4) The mass flow between stages can be reduced (deeper staging);

5) The volumetric utilization between stages can be increased (e.g.,swirl);

6) The pressure can be increased;

7) The density can be increased.

To increase the reducing potential in the flue gases, several methodsare available:

1) The temperature can be increased;

2) The stoichiometric ratio (i.e., the air-to-fuel ratio) can bedecreased;

3) The local fuel flow can be increased (for fixed air flow);

4) The local air flow can be decreased (for fixed fuel flow).

Mixing within a stage also influences the reduction process. A perfectlymixed stage with a stoichiometric mixture is the best, since thesereaction conditions will give the highest temperature, while stillmaintaining the reducing environment; i.e., minimizing oxidationradicals like O radicals. But, since perfect mixing is impractical, inpractice a stoichiometric ratio less than one is used, which minimizesthe occurrence of localities with a stoichiometric ratio greater thanone. However, as mixing is reduced, a longer residence time and/orhigher temperature is needed to achieve a similar reduction of the totalacidity, acid dewpoint temperature, and/or SO₃ concentration. However,the temperature of the combustion gases is dependent to a certain extenton the level of mixing, going down if mixing is decreased. Therefore, ifan increased temperature is desired for a given degree of mixing,temperature must be increased by other means, such as preheating air,changing heat transfer characteristics of furnace, and the like.Alternatively or additionally, the residence time in the reducingenvironment can be increased by delaying lean stage air introduction,such as OFA injection.

Note that SO₃ is formed by the oxidation of SO₂ in a catalyst because acatalyst enables the oxidation of SO₂ though the following reaction:SO₂½O₂→SO₃

Production of SO₃ in a catalyst is independent of the SO₃ concentrationin the gas, since the catalyzed reaction is only dependent on the SO₂and O₂ concentration. Therefore, any SO₃ that is reduced by the presentinvention independently reduces the exit SO₃ and is not affected by anddoes not affect SO₃ production in a catalyst.

The present invention thus provides a method for controlling andreducing flue gas acidity, specifically the flue gas concentrations ofSO₃, in order to beneficially (1) affect the efficiency of anelectrostatic precipitator, and more particularly, (2) to reduce theconcentration of SO₃ and other reducible acids in the flue gas in orderto reduce the flue gas acidity and acid dewpoint, thereby reducing airheater pluggage, duct corrosion, and SO₃ emissions to the environment,which can be a source of visible plumes and localized acid rain.

In a preferred embodiment of the present invention, macro-staging toregulate furnace acidity and SO₃ levels is achieved through the use ofOFA. In another preferred embodiment, micro-staging to regulate furnaceacidity and SO₃ levels is achieved through the use of low-NOx burners.In yet another preferred embodiment, macro-staging and micro-stagingthrough the use of OFA and low-NOx burners in combination are used toregulate furnace acidity and SO₃ levels. For furnaces with SCRs inoperation, the acidity is preferably regulated to reduce total flue gasacidity. For furnaces without SCRs or with by-passed SCRs, the SO₃ ispreferably regulated such that the SO₃ levels going to the ESP enhanceor favor precipitation. For current ESPs, SO₃ levels between about 10 toabout 15 ppm (by volume) in the exhaust is desirable for best ESPefficiency.

The dewpoint temperature is a convenient parameter for estimating and/oradjusting the reducing environment variables in order to achieveadequate reduction of acidity and/or desired SO₃ levels. For a desiredlevel of SO₃ and operating relative humidity, the dewpoint can bedetermined and the reducing environment variable adjusted accordingly toachieve the desired dewpoint. Other methods of determining acidityand/or SO₃ level can be used for the same purpose without departing fromthe scope of the invention.

In a preferred embodiment of the present invention, a power plant isoperated to provide a deeply-staged, micro-stage or macro-stage reducingenvironment in the lower furnace. The OFA in the upper furnace providesthe necessary oxygen to ensure an acceptable level of burnout of theremaining unburned fuel, combustion intermediates, and CO. Additionally,an SCR can be used to reduce NOx. Thus, an embodiment of the presentinvention includes a combustion furnace with OFA and low NOx burners foruse with sulfur containing fuels to lower the dewpoint temperature andto reduce the SO₃ concentration. Additionally, an SCR can be provided toreduce NOx. The low NOx burners are preferably of a grade that providesadequate mixing in the primary stage to provide adequate acid dewpointtemperature lowering and SO₃ concentration reduction, thus permittingthe use of an SCR, if necessary. Thus, an embodiment of the presentinvention includes a combustion furnace with high-grade low NOx burnersfor the purpose of reducing the flue gas acidity, lowering the aciddewpoint temperature and reducing the flue gas SO₃ concentration. Thisembodiment can further include an SCR.

An adequate reducing environment according to the present invention isone that will reduce SO3 to SO2 in less than about 2 seconds, morepreferably, in less than about 0.5 seconds. In the present invention,such a reducing environment can be achieved when the first stage fluegas temperature is greater than or equal to 900 Kelvin (1160 degreesF.), more preferably greater than about 1255 K (1800 degrees F.), evenmore preferably greater than about 1650 K (2500 degrees F.). A reducingenvironment is one where the ratio of the concentrations of reducingradicals to oxidizing radicals is greater than about 1; morespecifically, the ratio of the concentrations of H radicals to Oradicals is greater than about 1. A better reducing environment is onewhere the ratio of the concentrations of reducing radicals to oxidizingradicals is greater than about 10; more specifically, the ratio of theconcentrations of H radicals to O radicals is greater than about 10.

Thus, a combustion furnace operated according to the present inventioninvolves the steps of:

-   a) partially combusting the fuel in a first stage to create a    reducing environment;-   b) maintaining the reducing environment for a sufficient time period    such that SO₃ is reduced to SO₂ to achieve a desirable level of SO₃;-   c) combusting the remainder of the fuel and combustion intermediates    in a second stage with oxidizing environment;    thereby controlling the levels of SO₃ in the flue gases.

A method according to the present invention for reducing or controllingSO₃ in a combustion process of a sulfur-containing fuel, includes thesteps:

-   a) partially combusting the fuel in a first stage to create a    reducing environment;-   b) maintaining the reducing environment for a sufficient time period    such that SO₃ is reduced to SO₂ to achieve a desirable level of SO₃;-   c) combusting the remainder of the fuel and combustion intermediates    in a second stage with oxidizing environment;    thereby reducing or controlling the levels of SO₃ in the flue gases.

These methods can include the step of micro-staging and/or macro-stagingthe first stage fuel combustion and or macro. The micro-staging can beprovided through the use of low-NOx burners and the macro-stagingthrough the use of over-fired air. The fuel can be any fuel, especiallycarbonaceous fuels such as coal.

EXAMPLES

The following examples illustrate the results that can be achieved usingmethods according to the present invention. Methods according to thepresent invention were used to reduce SO₃ emissions at 3 different powerplants. The experimental data shown in Tables 1 and 2 were achievedthrough the use of high-velocity over-fired air and were measured bythird-party companies. TABLE 1 Effects of Staging Depth on SO3 levels at2 different plants. Plant 1 Plant 2 Staging Depth Parameters ShallowDeep Shallow Deep Load (MW _(net)) 182 179 154    154 NOx (lb/MMBtu)0.64 0.36 0.63 0.28 Coal % S (%) 1.22 1.22 0.87 0.87 Outlets SO₂ (ppm)1100 1100 720    720 Outlet SO₃ (ppm) 19 5.7 11*   0.5 SO₃/SO₂ (%) 1.70.52 1.5* 0.07 SO₃ Reduction 70% 95%*Estimated based on assumption that 98.5% of the sulfur in coal goes toSO2 and 1.5% of the sulfur in coal goes to SO3.

For the “shallow” staging cases, the over-fired air ports were nearlyclosed, but still contained cooling flow (around 10% of the total air).For the “mid” staging case, the over-fired air ports made up nearly 20%of the total air flow. For the “deep” staging cases, the over-fired airports made up nearly 30% of the total air flow. All three units werecorner-fired units and the OFA system was located well above the burnerzone. TABLE 2 Effect of three levels of staging at a single plant (Plant3, different from Table 1). Staging depth Parameters Shallow Mid DeepLoad (MW _(net)) 72 72 72 NOx (lb/MMBtu) 0.56 0.48 0.34 Coal % S (%)2.85 2.85 2.85 Outlets SO₂ (ppm) 1856 1855 1856 Outlet SO₃ (ppm) 5.9 1.91.1 SO₃/SO₂ (%) 0.32 0.1 0.06 SO₃ Reduction 68% 81% (vs Shallow)

Thus, the experimental data demonstrate the ability to regulate SO₃levels using methods according to the present invention.

Certain modifications and improvements will occur to those skilled inthe art upon a reading of the foregoing description. All modificationsand improvements have been deleted herein for the sake of concisenessand readability but are properly within the scope of the followingclaims.

1. A method for reducing SO₃ in a combustion process of asulfur-containing fuel, the method steps comprising: a) partiallycombusting the fuel in a first stage to create a reducing environment;b) maintaining the reducing environment for a sufficient time periodsuch that SO₃ is reduced to SO₂ to achieve a desirable level of SO₃; c)combusting the remainder of the fuel and combustion intermediates in asecond stage with oxidizing environment; thereby reducing the levels ofSO₃ in the flue gases.
 2. The method of claim 1, further including thestep of micro-staging the first stage fuel combustion.
 3. The method ofclaim 2, wherein the micro-staging is provided through the use oflow-NOx burners.
 4. The method of claim 1, further including the step ofmacro-staging the first stage of fuel combustion.
 5. The method of claim4, wherein the macro-staging is provided through the use of over-firedair.
 6. The method of claim 1, further including a combination ofmicro-staging and macro-staging.
 7. The method of claim 6, wherein themicro-staging is provided by low-NOx burners and the macro-staging isprovided by over-fired air.
 8. The method of claim 1, wherein the fuelis coal.
 9. A combustion furnace operated with a method for controllingSO₃ in a combustion process of a sulfur-containing fuel, the methodsteps comprising: a) partially combusting the fuel to create a reducingenvironment; b) maintaining the reducing environment for a sufficientperiod such that SO₃ is reduced to SO₂ to achieve a desirable level ofSO₃; c) combusting the remainder of the fuel in an oxidizingenvironment; thereby reducing the conversion of levels of SO₃ in theflue gases.
 10. The method of claim 9, further including the step ofmicro-staging the first stage fuel combustion.
 11. The method of claim10, wherein the micro-staging is provided through the use of low-NOxburners.
 12. The method of claim 9, further including the step ofmacro-staging the first stage of fuel combustion.
 13. The method ofclaim 12, wherein the macro-staging is provided through the use ofover-fired air.
 14. The method of claim 9, further including acombination of micro-staging and macro-staging.
 15. The method of claim14, wherein the micro-staging is provided by low-NOx burners and themacro-staging is provided by over-fired air.
 16. The method of claim 9,wherein the fuel is coal
 17. A method for controlling SO₃ concentrationsin a combustion process of a sulfur-containing fuel, the method stepscomprising: a) partially combusting the fuel in a first stage to createa reducing environment; b) adjusting the reducing environment timeperiod such that SO₃ is preferentially reduced to SO₂ to achieve adesirable level of SO₃; c) combusting the remainder of the fuel andcombustion intermediates in a second stage with oxidizing environment;thereby controlling the levels of SO₃ in the flue gases.
 18. The methodof claim 17, further including the step of micro-staging the first stagefuel combustion.
 19. The method of claim 18, wherein the micro-stagingis provided through the use of low-NOx burners.
 20. The method of claim17, further including the step of macro-staging the first stage of fuelcombustion.
 21. The method of claim 20, wherein the macro-staging isprovided through the use of over-fired air.
 22. The method of claim 17,further including a combination of micro-staging and macro-staging. 23.The method of claim 22, wherein the micro-staging is provided by low-NOxburners and the macro-staging is provided by over-fired air.
 24. Themethod of claim 17, wherein the fuel is coal.